Systems for transmitting many low-speed data as high-speed data based upon a time division multiplex system can be divided into a character multiplex system for multiplexing the data giving attention to characters, and a bit multiplex system for multiplexing the data based upon bit units. According to the former system, the data are multiplexed after the characters, sandwiched by a start bit and a stop bit, are received. Therefore, there inevitably develops a delay of two to three characters during the transmission and reception. Hence, this system is not suited for applications in which a minimum of delay is required, such as the transmission of telex exchange signals. Accordingly, CCITT is employing a multiplex system based upon bit units, and has already issued a Recommendation R101.
With a time division multiplex transmission apparatus of this kind much attention was given to the economic aspect. Thus it was accepted practice to use a single circuit by concentrating the same processing functions at low-speed channels.
The above-mentioned circuit setup for the conventional time division multiplex transmission apparatus is capable of providing a relatively economical system when the speed of input data and the character constructions are the same or are very restricted, when all of the l channels are to be utilized, or when the reliability of the whole system is not a major concern.
However, it is difficult to construct the start-stop regenerating circuit with hardware when it is necessary to combine the data signals of a speed between 50 to 300 Baud having a character structure of 7.5 to 11 units or when the combination is likely to be changed, as requested by the Alternative B mentioned in the Recommendation R101 of CCITT.
Even when the above-mentioned portion is processed by a software technique, too many channels impose a limitation on the processing speed of a general-purpose processor unless an expensive and a specially designed high-speed processor is used. Furthermore, when the program of just one channel is to be changed, it is necessary to change the whole program.
Furthermore, the increase in the scale of the circuit of a common portion inevitably decreases the reliability of the system. When it is necessary to maintain high reliability, it is necessary to install a double-circuit system causing the scale of the apparatus to be increased and also causing the initial setup cost, with less channel numbers, to become expensive.